Method for recording information on optical recording medium and information recording apparatus

ABSTRACT

According to the invention, a record mark is formed by: using one ON-pulse pattern and one ON-pulse pattern following it when the record mark to be formed is shorter than a specified length X (nT&lt;X); and using two ON-pulse patterns and two ON-pulse patterns respectively following them when the record mark to be formed is longer than the specified length X (nT&gt;X). This makes is possible to secure a heat amount per unit time sufficiently in forming a short record mark even when a targeted recording linear velocity is high. Also, an unwanted heat pocket becomes hard to arise in the record layer even when a sufficient heat amount per unit time is secured in forming a long record mark. Therefore, it becomes possible to form a record mark having a good shape even when data is recorded at a high linear velocity.

BACKGROUND OF THE INVENTION

The present invention relates to a method for recording information onan optical recording medium, especially a method for recordinginformation on a write-once type optical recording medium. Also, theinvention relates to an information recording apparatus for recordinginformation on an optical recording medium, especially an informationrecording apparatus for recording information on a write-once typeoptical recording medium.

In recent years, there have been proposed next-generation opticalrecording media which have a much larger recording capacity incomparison to conventional ones and which allow the achievement of anextremely higher data transfer rate, and some of them have come intopractical use (see Patent Document 1, JP-A-2003-203383). Unlike aconventional optical recording medium, with such next-generation opticalrecording media, a laser beam of a wavelength of about 205 nm and anobjective having a numerical aperture of about 0.85 are used for datarecording and reproduction. This allows the beam spot diameter of alaser beam to be narrowed down to about 0.39 μm in a plane of recording,thereby achieving a recording capacity of about 25 GB/side and a datatransfer rate of about 36 Mbps at a reference linear velocity (about 4.9m/sec).

Also, in regard to next-generation optical recording media, varioustypes of optical recording media such as read-only type, write-oncetype, and rewritable type have been proposed like existing opticalrecording media including CDs (Compact Disc) and DVD (Digital VersatileDisc). However, of these media, write-once type ones have been known tohave the feature that a heat amount per unit time required for datarecording is increased with an increase in targeted recording linearvelocity. In order to increase a heat amount per unit time, it isnecessary to use a higher-power semiconductor laser, or to furtherlengthen the length of an ON-pulse pattern, i.e. the time to set theintensity of a laser beam at a recording-power level.

[Patent Document 1] JP-A-2003-203383

However, a laser beam to be used for next-generation optical recordingmedia is a light in a blue-color wavelength region as described above,and a semiconductor laser capable of generating such laser beam at ahigh output power is very expensive. Hence, in order to increase a heatamount to be applied per unit time thereby to enable recording at anextremely high linear velocity e.g. a fourfold data rate, it is usefulto lengthen the length of an ON-pulse pattern, i.e. the time to set theintensity of a laser beam at a recording-power level as far as possible.Therefore, adopting a pulse train pattern (i.e. so-called solid pattern)in which one ON-pulse pattern is used to form a record mark can make thetime to set the intensity of a laser beam at a recording-power levellongest. However, using such pulse train pattern poses a problem suchthat an unwanted heat pocket is produced in a record layer in forming along record mark, thereby causing a thermal interference and thusdeteriorating recording properties.

SUMMARY OF THE INVENTION

Therefore, it is an object of the invention to provide a method forrecording information on a write-once type optical recording medium,which is suitable for data recording at a high linear velocity.

Also, it is another object of the invention to provide an informationrecording apparatus for recording information on a write-once typeoptical recording medium, which enables data recording at a high linearvelocity.

The method for recording information of the invention is an informationrecording method for recording information by irradiating a write-oncetype optical recording medium having a base and at least one recordlayer on the base with a laser beam modulated according to a pulse trainpattern including at least a recording-power level, a base power level,and a medium power level having a power intermediate between therecording-power and base power levels to form a record mark in therecord layer, wherein the pulse train pattern includes at least oneON-pulse pattern and at least one OFF-pulse pattern, the ON-pulsepattern transitions from one of the base power level and the mediumpower level to the recording-power level, and then transitions from therecording-power level to the base power level, the OFF-pulse patternincludes the base power level following the ON-pulse pattern, and whenthe record mark longer than a specified length is formed, the number ofthe at least one ON-pulse pattern to be used is two, and a length of theOFF-pulse pattern following the last ON-pulse pattern is made constantregardless of a length of the record mark to be formed.

The information recording apparatus of the invention is an informationrecording apparatus for recording information by irradiating awrite-once type optical recording medium having a base and at least onerecord layer on the base with a laser beam modulated according to apulse train pattern including at least a recording-power level, a basepower level, and a medium power level having a power intermediatebetween the recording-power and base power levels to form a record markin the record layer, the apparatus arranged so that the pulse trainpattern includes at least one ON-pulse pattern and at least oneOFF-pulse pattern, the ON-pulse pattern transitions from one of the basepower level and the medium power level to the recording-power level, andthen transitions from the recording-power level to the base power level,the OFF-pulse pattern includes the base power level following theON-pulse pattern, and when the record mark longer than a specifiedlength is formed, the number of the at least one ON-pulse pattern to beused is two, and a length of the OFF-pulse pattern following the lastON-pulse pattern is made constant regardless of a length of the recordmark to be formed.

According to the invention, the number of the ON-pulse patterns is twoin forming a long mark, which makes an unwanted heat pocket hard toarise in the record layer. Thus, it becomes possible to form a recordmark having a good shape. Moreover, because the length of the OFF-pulsepattern following the last ON-pulse pattern is made constant, the degreeof the thermal interference with a record mark to be formed subsequentlyis rendered almost constant regardless of the kind of the record markand thus a deviation in the length of the space region between a recordmark in question and another record mark to be recorded subsequentlybecomes less prone to being caused.

In the invention, it is preferable that the number of the ON-pulsepatterns in forming a record mark shorter than the specified length isone. When the number of ON-pulse patterns in forming a short mark inthis way is one, i.e. the pulse pattern is a solid pattern, it becomespossible to secure a heat amount per unit time sufficiently even with ahigh targeted recording linear velocity and therefore a record markhaving a good shape can be formed.

In the invention, it is preferable that the length of the OFF-pulsepattern located between the two ON-pulse patterns is made longer as thelength of the record mark to be formed is longer when a record marklonger than the predetermined length is formed. This makes possible toeffectively prevent a heat pocket from taking place when a long markeasy to produce a heat pocket is formed.

In the invention, it is preferable thatnT*0.70≦t _(total) ≦nT*0.85,where a sum of the lengths of the two ON-pulse patterns used in formingthe record mark longer than the specified length is t_(total), and thelength of the record mark to be formed is nT (T is a clock cycle). Thismakes a heat amount to be applied optimal and as such, it becomespossible to form a record mark having a good shape.

In the invention, it is preferable thatt _(total)*0.45≦t _(top) ≦t _(total)*0.65,where a length of first applied one of the two ON-pulse patterns used informing the record mark longer than the specified length is t_(top).This is because when the difference in length between the two ON-pulsepatterns is too large, heating by the shorter ON-pulse pattern doesn'twork out effectively, deteriorating the jitter.

In the invention, it is preferable that, of the two ON-pulse patternsused in forming the record mark longer than the specified length, alength of the OFF-pulse pattern following the last ON-pulse pattern isfrom 0.4T to 0.5T inclusive. The reason for this is twofold. The firstis that when the length of the OFF-pulse pattern following the secondON-pulse pattern is made too long, the length of the first OFF-pulsepattern must be shortened accordingly, thereby excessively reducing thekinds of record marks which can be formed using the two ON-pulsepatterns. The second is that when the length of the OFF-pulse patternfollowing the second ON-pulse pattern is made too short reversely, thelength of the first OFF-pulse pattern must be made longer, whereby thefirst OFF-pulse pattern could cause the record mark to dispart.

In the invention, it is preferable that the specified length is0.82*λ/NA, where a wavelength of the laser beam is λ, a numericalaperture of an objective for condensing the laser beam is NA. The lengthgiven by 0.82*λ/NA represents to the diameter of a beam spot. Thisavoids that the intensity of the laser beam 50 is fixed at therecording-power level Pw over all the periods when the beam spot goesthrough a certain point. As a result, it becomes possible to prevent theoccurrence of an excessive heat pocket.

In the invention, it is preferable that the following condition issatisfied:λ/NA≦640 nm,where a wavelength of the laser beam is λ, and a numerical aperture ofan objective for condensing the laser beam is NA. The reason for this isthat the recording linear velocity must be set higher because a systemincorporating an optical system like this requires a remarkably highdata transfer rate.

In the manner as stated above, the invention makes possible not only tosufficiently secure a heat amount per unit time even when a targetedrecording linear velocity is high, but also to form a record mark havinga good shape even when the mark to be formed is long because an unwantedheat pocket is hard to arise in the record layer. Moreover, the degreeof the thermal interference with a record mark to be formed subsequentlyis rendered almost constant regardless of the kind of the record markand as such, a deviation in the length of the space region between arecord mark in question and another record mark to be recordedsubsequently becomes less prone to being caused.

Therefore, the invention allows good signal characteristics to beobtained even when data is recorded at a high linear velocity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views showing a structure of an optical recordingmedium 10 according to a preferred embodiment of the invention. Morespecifically, FIG. 1A is a perspective view, partially broken away, andFIG. 1B is a fragmentary sectional view showing the part A shown in FIG.1A in a magnified form.

FIGS. 2A and 2B are views showing specific pulse train patterns for alaser beam 50 in recording. Of the drawings, FIG. 2A shows a pulse trainpattern when a record mark shorter than the specified length X isformed, and FIG. 2B shows a pulse train pattern when a record marklonger than the specified length X is formed.

FIG. 3 is a graph showing the results of Characterizations 1 and 2.

FIG. 4 is a graph showing the result of Characterization 3.

FIG. 5 is a graph showing the result of Characterization 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the invention will be described in detailbelow in reference to the accompanying drawings.

FIG. 1A is a perspective view, partially broken away, and it shows theappearance of an optical recording medium 10 according to a preferredembodiment of the invention. FIG. 1B is a fragmentary sectional viewshowing the part A shown in FIG. 1A in a magnified form.

As shown in FIG. 1A, the optical recording medium 10 according to theembodiment is a disc-like optical recording medium measuring about 120mm in outer diameter, and about 1.2 mm in thickness. The opticalrecording medium 10 includes: a supporting base 11; a light-transmissivelayer 12; and a record layer 13 provided between the supporting base 11and light-transmissive layer 12, as shown in FIG. 1B. The opticalrecording medium 10 according to the embodiment is a write-once typeoptical recording medium such that data can be recorded thereon andreproduced therefrom by applying a laser beam 50 having a wavelength λof 380 to 450 nm, more preferably about 405 nm to a surface of thelight-transmissive layer 12, i.e. the light-incident surface 12 a. Indata recording and reproduction on the optical recording medium 10, anobjective 51 having a numerical aperture of 0.65 or larger, morepreferably about 0.85 is used, thereby making the setting, λ/NA≦640 nm,where the wavelength of the laser beam 50 is λ, and the numericalaperture of the objective 51 is NA.

The supporting base 11 is a disc-like base having a thickness of about1.1 mm used to secure the thickness (about 1.2 mm) required for theoptical recording medium 10. On one surface of the supporting base, aland 11 a and a groove 11 b, both for guiding a laser beam 50, areformed in spiral patterns whirling from a vicinity of a center portionthereof toward an outer edge portion or the reverse direction, or formedin concentric circles. As the material for the supporting base 11,various materials may be used. For example, such materials includeglass, ceramics, and resins. Of these materials, resins are morepreferable from the viewpoint of the ease of forming. Such resinsincludes polycarbonate resin, olefin resin, acrylic resin, epoxy resin,polystyrene resin, polyethylene resin, polypropylene resin, siliconeresin, fluorine-based resin, ABS resin, and urethane resin. Of theseresins, polycarbonate resin and olefin resin are especially preferablefrom the viewpoints of the ease of processing, or others. However, thesupporting base 11 doesn't necessarily have a high optical transparencybecause the supporting base 11 is not located on the optical path of thelaser beam 50.

For preparing the supporting base 11, it is preferable to use theinjection molding in which a stamper is used. However, the supportingbase 11 may be prepared by another method such as a photopolymer (2P)technique.

The light-transmissive layer 12 is a layer constituting a part of theoptical path of the laser beam 50, the thickness of which is preferablyset within a range of 10 to 200 μm, and more preferably, it is set atabout 100 μm. The material for the light-transmissive layer 12 is notparticularly limited as long as it is a material having a lighttransmissivity sufficiently high in a wavelength band of a laser beam 50to be used. However, it is preferable to use an acrylic or epoxy-basedultraviolet-curing resin as such material. Alternatively, thelight-transmissive layer 12 may be formed by using a light-transmissivesheet of a light-transmissive resin and an adhesive agent or a tackinessagent instead of the film formed by curing an ultraviolet-curing resin.

The record layer 13 is a layer in which an irreversible record mark isto be formed by irradiation of the laser beam 50. While variousmaterials may be used as the material for the record layer 13, it ispreferable to use a material composed of: a dielectric base materialcontaining, as its main component, an admixture of ZnS and SiO₂ orLaSiON (i.e. an admixture of La₂O₃, SiO₂, and Si₃N₄); and magnesiumand/or aluminum added in the dielectric base material.

When an admixture of ZnS and SiO₂ is used as the main component of thedielectric base material included by the record layer 13, it ispreferable to set the molar ratio of ZnS vs. SiO₂ within a range of50:50 to 90:10, and most preferably at 80:20 approximately. In addition,when LaSiON is used as the main component of the dielectric basematerial, it is preferable to set the molar ratio of SiO₂ vs. sum ofSi₃N₄ and La₂O₃ within a range of 10:90 to 50:50. While “main component”herein implies that the percentage of a material in question (theadmixture of ZnS and SiO₂ or LaSiON) in the dielectric base material is50 atomic percent (atomic %) or larger, it is especially preferable thatthe percentage is 80 atomic % or larger. It is preferable that theamount of magnesium and/or aluminum to be added in the dielectric basematerial is set within a range of 18 to 55 atomic %, especiallypreferable within a range 20 to 35 atomic %, and most preferable at 25atomic %.

The structure of the optical recording medium 10 according to theembodiment is as described above, in which a record mark can be formedin the record layer 13 by applying an intensity-modulated laser beam 50to the record layer 13 from the side of the light-incident surface 12 a.The length of each record mark and the length (of a space) betweenrecord marks (i.e. between edges) are set to be multiple of a clockcycle T, i.e. nT (n=integer). For example, when (1,7) RLL modulationsystem is adopted, the lengths of each record mark and the space are setat any of 2T to 8T.

However, in order to perform the recording on such optical recordingmedium 10 at a high linear velocity, a heat amount required per unittime is increased. For increase in the heat amount per unit time, it isuseful to use a pulse train pattern (solid pattern) by which oneON-pulse pattern is used to form one record mark. However, as alreadydescribed, use of such pulse train pattern produces an unwanted heatpocket in the record layer 13 in forming a long record mark, therebycausing a thermal interference and thus deteriorating recordingproperties. Especially, the optical recording medium 10 according to theembodiment includes no reflection film as shown in FIG. 1B and as such,the recording medium is hard to dissipate the heat produced by the laserbeam 50 and remarkably easy to have a heat pocket caused therein.

Allowing for the foregoing, in the invention, one ON-pulse pattern andone OFF-pulse pattern following it are used in forming a record markshorter than the specified length (=X), and two ON-pulse patterns andtwo OFF-pulse patterns respectively following them are used in forming arecord mark longer than the specified length.

FIGS. 2A and 2B are views showing specific pulse train patterns for alaser beam 50 in recording. Of the drawings, FIG. 2A shows a pulse trainpattern when a record mark shorter than the specified length X isformed, and FIG. 2B shows a pulse train pattern when a record marklonger than the specified length X is formed.

As shown in FIG. 2A, in a case where a record mark to be formed isshorter than the specified length X (nT<X), the intensity 50 a of alaser beam is first set at a medium power level Pm. After that, at Timet₁₁ the laser beam intensity is changed from the medium power level Pmto recording-power level Pw, and then it is changed from therecording-power level Pw to base power level Pb at Time t₁₂, withproviso that the change in the intensity of a laser beam takes a certainlength of transition time and as such, “Time” herein refers to a timingat which the intensity change reaches the midpoint between apre-transition level and a post-transition level.

The period during which the intensity 50 a of the laser beam 50 is setat the recording-power level Pw coincides with a period during which aheat required for the formation of a record mark is applied, and“ON-pulse pattern” refers to a waveform portion corresponding to suchperiod. More specifically, it refers to a waveform portion during whichthe intensity 50 a of the laser beam 50 is caused to transition from thebase power level Pb or medium power level Pm to the recording-powerlevel Pw, and subsequently from the recording-power level Pw to the basepower level Pb. The length of ON-pulse pattern is defined by a periodfrom the time when the intensity 50 a of the laser beam 50 reaches themidpoint between a pre-rising level (i.e. the base power level Pb ormedium power level Pm) and a post-rising level (i.e. the recording-powerlevel Pw) to the time when the intensity 50 a reaches the midpointbetween a pre-decreasing level (i.e. the recording-power level Pw) and apost-decreasing level (i.e. the base power level Pb).

As for the pulse train pattern shown in FIG. 2A, the waveform of theperiod t_(top) from Time t₁₁ to Time t₁₂ makes an ON-pulse pattern andtherefore the number of ON-pulse patterns is one. In addition, theperiod during which the intensity 50 a of the laser beam is set at thebase power level Pb corresponds to a period to cool the record layer 13.A waveform portion corresponding to such cooling period is referred toas “OFF-pulse pattern.” In regard to the pulse train pattern shown inFIG. 2A, the waveform of the period t_(cl) from Time t₁₂ to Time t₁₃makes an OFF-pulse pattern. More specifically, it refers to a waveformportion during which the intensity 50 a of the laser beam 50 is causedto transition from the recording-power level Pw to the base power levelPb, and subsequently from the base power level Pw to the recording-powerlevel Pw or medium power level Pm. The length of OFF-pulse pattern isdefined by a period from the time when the intensity 50 a of the laserbeam 50 reaches the midpoint between a pre-decreasing level (i.e. therecording-power level Pw) and a post-decreasing level (i.e. the basepower level Pb) to the time when the intensity 50 a reaches the midpointbetween a pre-rising level (i.e. the base power level Pb) and apost-rising level (i.e. the recording-power level Pw or medium powerlevel Pm).

As described above, in a case where a record mark to be formed isshorter than the specified length X (nT<X), the number of ON-pulsepatterns is one, i.e. “solid pattern” is used and. On this account, itbecomes possible to secure a sufficient heat amount per unit time evenwhen the targeted recording linear velocity is high. Also, it becomespossible to form a record mark having a good shape.

On the other hand, as shown in FIG. 2B, in a case where a record mark tobe formed is longer than the specified length X (nT>X), the intensity 50a of a laser beam is first set at the medium power level Pm. After that,at Time t₂₁ the laser beam intensity is changed from the medium powerlevel Pm to recording-power level Pw, and then it is changed from therecording-power level Pw to base power level Pb at Time t₂₂. Then, afterthe laser beam intensity is changed from the base power level Pb to therecording-power level Pw again at Time t₂₃, it is changed from therecording-power level Pw to the base power level Pb at Time t₂₄.

As for the pulse train pattern shown in FIG. 2B, the waveforms of theperiod t_(top) from Time t₂₁ to Time t₂₂ and the period t_(last) fromTime t₂₃ to Time t₂₄ make ON-pulse patterns and therefore the number ofON-pulse patterns is two. In addition, the period between the ON-pulsepatterns, during which the intensity 50 a of the laser beam is set atthe base power level Pb, corresponds to a period to cool the recordlayer 13, i.e. makes a first OFF-pulse pattern. In regard to the pulsetrain pattern shown in FIG. 2B, the waveform of the period t_(off) fromTime t₂₂ to Time t₂₃ makes the second OFF-pulse pattern. The secondOFF-pulse pattern is the waveform of the period t_(cl) from Time t₂₄ toTime t₂₅. In the pulse train pattern shown in FIG. 2B, the length t_(cl)of the second OFF-pulse pattern is made constant regardless of thelength of a record mark to be formed.

As described above, in a case where a record mark to be formed is longerthan the specified length X (nT>X), the number of ON-pulse patterns istwo, and one OFF-pulse pattern is located between the ON-pulse patterns.On this account, an unwanted heat pocket becomes hard to arise in therecord layer even when a sufficient heat amount per unit time issecured, and thus it becomes possible to form a record mark having agood shape. In addition, the length t_(cl) of the second OFF-pulsepattern is made constant and as such, the degree of the thermalinterference with a record mark to be formed subsequently is renderedalmost constant regardless of the kind of the record mark and thus adeviation in the length of the space region between a record mark inquestion and another record mark to be recorded subsequently becomesless prone to being caused. In other words, it becomes possible toimprove the jitter of a space region (i.e. space jitter).

Also, for the pulse train pattern shown in FIG. 2B, it is preferablethat the longer the length of a record mark to be formed is, the longerthe length t_(off) of the first OFF-pulse pattern located between thetwo ON-pulse patterns is set to be. This makes it possible to preventthe occurrence of a heat pocket effectively in forming a longer mark,which is prone to cause a heat pocket.

Further, when the sum of the lengths of the two ON-pulse patterns,namely the sum of the periods t_(top) and t_(last), is represented byt_(total), it is preferable to set the t_(total) as follows:nT*0.70≦t _(total) ≦nT*0.85.It is more preferable to set the t_(total) as follows:nT*0.75≦t _(total) ≦nT*0.80.This makes a heat amount to be applied optimal and as such, it becomespossible to form a record mark having a good shape.

In regard to the relation between the length t_(top) of the ON-pulsepattern to be applied first and the length t_(last) of the ON-pulsepattern to be applied last, when the sum of the periods t_(top) andt_(last) is represented by t_(total) as described above, it ispreferable to make the setting as follows:t _(total)*0.45≦t _(top) ≦t _(total)*0.65.It is more preferable to make the setting as follows:t _(total)*0.50≦t _(top) ≦t _(total)*0.60.This is because when the difference in length between the two ON-pulsepatterns is too large, heating by the shorter ON-pulse pattern doesn'twork out effectively, deteriorating the jitter.

Further, as for the length t_(cl) of the second OFF-pulse pattern usedin forming a record mark with two ON-pulse patterns, it is preferable toset the length within a range from 0.4T to 0.5T inclusive. The reasonfor this is twofold. The first is that when the length t_(cl) of thesecond OFF pulse pattern is made too long, the length t_(off) of thefirst OFF-pulse pattern must be shortened accordingly, therebyexcessively reducing the kinds of record marks which can be formed usingthe two ON-pulse patterns. The second is that when the length t_(cl) ofthe second OFF-pulse pattern is made too short reversely, the lengtht_(off) of the first OFF-pulse pattern must be made longer, whereby thefirst OFF-pulse pattern could cause the record mark to dispart.

Now, in regard to the specified length X, it is preferable to set thelength X at a length given by 0.82*λ, i.e. the length corresponding tothe diameter of a beam spot, where λ is the wavelength of the laser beam50, and NA is the numerical aperture of the objective 51 for condensingthe laser beam 50. This is based on the consideration concerning thefact that continuously irradiating the record layer 13 with the laserbeam 50 at the recording-power level Pw makes the temperature of therecord layer 13 highest in the location where a trailing edge portion ofthe beam spot impinges on the record layer 13, whereby the portion ofthe record layer in that location is overheated. This implies that thelaser beam 50 with its intensity fixed at the recording-power level Pwover all the periods will cause an excessive heat pocket when the beamspot goes through a certain point. Therefore, when the diameter of thebeam spot is defined as “specified length X,” it becomes possible toprevent the occurrence of such heat pocket.

The specific pulse train patterns in the embodiment are as describedabove.

It is preferable that the information to determine such pulse trainpatterns is saved in the optical recording medium 10 as “information forsetting recording conditions.” If information for setting recordingconditions like this is saved in the optical recording medium 10, when auser executes the data recording actually, the information recordingapparatus can read out the information for setting recording conditionsto decide a pulse train pattern based on the information.

As for the information for setting recording conditions, it ispreferable that the information includes information required fordetermination of various conditions (e.g. recording linear velocity),which are requisite for data recording on the optical recording medium10, as well as pulse train patterns. The information for settingrecording conditions may be recorded in wobbles or prepits, or otherwiseit may be recorded in the record layer 13 as data. Also, the informationfor setting recording conditions may not only directly show variousconditions requisite for data recording but also specify any of thevarious conditions previously stored in the information recordingapparatus thereby to indirectly determine a pulse train pattern.

The invention is not limited the above embodiments, and various changesand modifications may be made within a scope of the invention stated inclaims. It is needless to say that such changes and modifications areincluded in the scope of the invention.

For example, while the optical recording medium 10 according to theabove embodiment is arranged so that the record layer 13 is directlysandwiched between the supporting base 11 and light-transmissive layer12, a dielectric layer may be provided on at least one of the top andbottom surfaces of the record layer 13 thereby to protect the recordlayer 13 physically and chemically.

EXAMPLE

An example of the invention will be described below. However, theinvention is not limited to the example in any way.

[Preparation of Test Samples]

Test samples each having the same structure as that of the opticalrecording medium 10 shown in FIG. 1 were prepared by the followingmethod.

First, a disc-shaped supporting base 11 of polycarbonate, measuringabout 1.1 mm in thickness, about 120 mm in diameter and having a land 11a and a groove 11 b formed on its surface was prepared by injectionmolding. The depth of the groove 11 b was set at about 21 nm, and thewidth thereof was set at about 169 nm. The track pitch was set at about320 nm.

Next, the supporting base 11 was loaded into a sputtering apparatus.Then, both a mixed target of ZnS and SiO₂ (whose molar ratio=80:20) anda target of magnesium were used to form a 36 nm-thick record layer 13 onthe surface of the base where the land 11 a and groove 11 b are formedby sputtering. The chemical compositions of Zn, Si, Mg, O, S containedin the record film 13 were 22.6, 9.3, 25.0, 18.6, and 24.5 atomic %respectively.

Then, the record layer 13 was coated with an ultraviolet-curing acrylicresin by spin-coating and irradiated with ultraviolet radiation therebyto form a light-transmissive layer 12 having a thickness of about 100μm. In this way, test samples were prepared.

[Evaluation 1]

The resulting test samples were loaded into an optical disc evaluatingapparatus (an optical disc evaluating apparatus DDU-1000 manufactured byPulstec Industrial, Co., Ltd.). Then, the samples were irradiated with alaser beam with a wavelength of about 405 nm from the side of thelight-incident surface 12 a toward the record layer 13 through anobjective with a numerical aperture of about 0.85 while being rotated ata linear velocity of about 19.7 m/s. During the operation, 1,7 RLL mixedsignals (2T-8T) were recorded.

In regard to pulse train patterns, the pulse train pattern shown in FIG.2A was used for the marks 2T to 4T, and the pulse train pattern shown inFIG. 2B was used for the marks 5T to 8T. The medium power level Pm wasset at 5.0 mW. The base power level Pb was set at 2.5 mW. Therecording-power level Pw was set within a range of 11.0 to 12.6 mWvariously. The lengths of t_(top), t_(last), t_(off), and t_(cl) wereset for each record mark as shown in Table 1.

TABLE 1 Record Mark t_(top) t_(off) t_(last) t_(cl) 2T 1.7T — — 0.1T 3T2.3T — — 0.4T 4T 3.1T — — 0.5T 5T 2.1T 0.2T 1.8T 0.5T 6T 2.6T 0.3T 2.2T0.5T 7T 2.9T 0.6T 2.6T 0.5T 8T 3.4T 1.0T 2.7T 0.5T

The recorded signals were reproduced at a linear velocity of about 4.9m/s and equalized in their waveform by an equalizer, followed bymeasuring jitters using a Time Interval Analyzer TA720 manufactured byYokogawa Electric Corp.

The results of the measurements are shown in FIG. 3. As shown in FIG. 3,the jitter becomes better as the recording-power level Pw is set at ahigher value. When the recording-power level Pw was set at 12.6 mW, aremarkably low value as low as about 6.1% was obtained.

[Evaluation 2]

The signal recording and reproduction and the measurement of jitterswere performed as in the case of Characterization 1 except that thepulse train pattern (solid pattern) shown in FIG. 2A was used to formall the record marks (2T to 8T), and the lengths of t_(top) and t_(cl)were set as shown in Table 2.

TABLE 2 Record Mark t_(top) t_(off) t_(last) t_(cl) 2T 1.7T — — 0.1T 3T2.3T — — 0.4T 4T 3.1T — — 0.5T 5T 4.0T — — 0.6T 6T 4.9T — — 0.7T 7T 5.9T— — 0.7T 8T 6.8T — — 0.8T

The results of the Characterization 2 are shown in FIG. 3 together withthose of Characterization 1. As shown in FIG. 3, it was confirmed thatthe jitter was worse in comparison with that obtained fromCharacterization 1.

[Evaluation 3]

The signal recording and reproduction and the measurement of jitterswere performed as in the case of Characterization 1 except that therecording-power level Pw was fixed at 12.6 mW, and when the marks 5T to8T were formed, t_(total) was made constant and the ratios of t_(top)and t_(last) were set at various values.

The results of the measurements are shown in FIG. 4. As shown in FIG. 4,it was confirmed that the jitter was less than or equal to 6.5% withinthe following range:t _(total)*0.45≦t _(top) ≦t _(total)*0.65.Also, it was confirmed that the jitter was the best within the followingrange:t _(total)*0.50≦t _(top) ≦t _(total)*0.60.[Evaluation 4]

The signal recording and reproduction were performed in conditions wherethe length t_(cl) of an OFF-pulse pattern required to fix, i.e. thesecond OFF-pulse pattern when two ON-pulse patterns are used, was setvariously. When t_(cl) was fixed at 0.4T, the recording was performedunder the conditions shown in Table 3. When t_(cl) was fixed at 0.6T,the recording was carried out under the conditions shown in Table 4.When t_(cl) was fixed at 0.7T, the recording was executed under theconditions shown in Table 5.

TABLE 3 Record Mark t_(top) t_(off) t_(last) t_(cl) 2T 1.7T — — 0.1T 3T2.3T — — 0.4T 4T 3.1T — — 0.4T 5T 2.0T 0.3T 1.9T 0.4T 6T 2.4T 0.6T 2.1T0.4T 7T 2.7T 1.0T 2.4T 0.4T 8T 3.0T 1.6T 2.5T 0.4T

TABLE 4 Record Mark t_(top) t_(off) t_(last) t_(cl) 2T 1.7T — — 0.1T 3T2.3T — — 0.4T 4T 3.1T — — 0.5T 5T 4.0T — — 0.6T 6T 2.6T 0.2T 2.3T 0.6T7T 3.0T 0.4T 2.7T 0.6T 8T 3.4T 0.7T 3.0T 0.6T

TABLE 5 Record Mark t_(top) t_(off) t_(last) t_(cl) 2T 1.7T — — 0.1T 3T2.3T — — 0.4T 4T 3.1T — — 0.5T 5T 4.0T — — 0.6T 6T 4.9T — — 0.7T 7T 3.1T0.2T 2.8T 0.7T 8T 3.5T 0.4T 3.2T 0.7T

Recorded signals were reproduced, followed by measuring space jitters.The results of the measurements are shown in FIG. 5. Also, FIG. 5 showsthe jitters obtained when the recording was performed under theconditions shown in Table 1 (t_(cl)=0.5T) together.

As shown in FIG. 5, the space jitter was the best when the length t_(cl)of the second OFF-pulse pattern was 0.5T (Characterization 1). Also,when the length t_(cl) of the second OFF-pulse pattern was 0.4T, arelatively good space jitter was obtained. However, it was confirmedthat the space jitter became worse as the length t_(cl) of the secondOFF-pulse pattern was made longer such as 0.6T and 0.7T. From above, ithas been confirmed that it is preferable to set the length t_(cl) of thesecond OFF-pulse pattern when two ON-pulse patterns are used, within arange of 0.4T to 0.5T. Particularly, it has been confirmed that it isthe most preferable to set the length t_(cl) at a value of about 0.5T.

While the invention has been described in detail and in reference to theparticular embodiments, it is apparent to those skilled in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the invention.

The application is based on Japanese Patent Application No. 2004-34368filed on Feb. 16, 2004, whose subject matter is incorporated herein byreference.

1. An information recording method for recording information,comprising: modulating a laser beam according to a pulse train patternincluding at least a recording-power level, a base power level, and amedium power level having a power intermediate between therecording-power and base power levels; and irradiating a write-onceoptical recording medium having a base and a record layer on the basewith the laser beam modulated according to the pulse train pattern toform a record mark in the record layer, wherein the pulse train patternincludes at least one ON-pulse pattern and at least one OFF-pulsepattern, the ON-pulse pattern transitions from one of the base powerlevel and the medium power level to the recording-power level, and thentransitions from the recording-power level to the base power level, theOFF-pulse pattern includes the base power level following the ON-pulsepattern, and when the record mark longer than a specified length isformed, the number of the at least one ON-pulse pattern to be used istwo, and a length of the OFF-pulse pattern following the last ON-pulsepattern is made constant regardless of a length of the record mark to beformed and a length of the OFF-pulse pattern located between the twoON-pulse patterns is made longer as the length of the record mark to beformed is longer, whereint _(total)*0.45≦t _(top) ≦t _(total)*0.65, where a length of firstapplied one of the two ON-pulse patterns used in forming the record marklonger than the specified length is t_(top), and a sum of the lengths ofthe two ON-pulse patterns used in forming the record mark longer thanthe specified length is t_(total).
 2. The information recording methodof claim 1, wherein when the record mark shorter than the specifiedlength is formed, the number of the at least one ON-pulse pattern isone.
 3. The information recording method of claim 1, wherein thespecified length is 0.82*λ/NA, where a wavelength of the laser beam isλ, a numerical aperture of an objective for condensing the laser beam isNA.
 4. The information recording method of claim 1, whereinnT*0.70≦t _(total) ≦nT*0.85, where the length of the record mark to beformed is nT (T is a clock cycle).
 5. The information recording methodof claim 1, wherein of the two ON-pulse patterns used in forming therecord mark longer than the specified length, a length of the OFF-pulsepattern following the last ON-pulse pattern is from 0.4T to 0.5Tinclusive (T is a clock cycle).
 6. The information recording method ofclaim 1, wherein the laser beam irradiation is performed under thefollowing condition:λ/NA≦640 nm, where a wavelength of the laser beam is λ, and a numericalaperture of an objective for condensing the laser beam is NA.
 7. Aninformation recording apparatus for recording information by irradiatinga write-once optical recording medium having a base and a record layeron the base, comprising: a laser configured to emit a laser beammodulated according to a pulse train pattern including at least arecording-power level, a base power level, and a medium power levelhaving a power intermediate between the recording-power and base powerlevels to form a record mark in the record layer, the apparatus arrangedso that the pulse train pattern includes at least one ON-pulse patternand at least one OFF-pulse pattern, the ON-pulse pattern transitionsfrom one of the base power level and the medium power level to therecording-power level, and then transitions from the recording-powerlevel to the base power level, the OFF-pulse pattern includes the basepower level following the ON-pulse pattern, and when the record marklonger than a specified length is formed, the number of the at least oneON-pulse pattern to be used is two, and a length of the OFF-pulsepattern following the last ON-pulse pattern is made constant regardlessof a length of the record mark to be formed and a length of theOFF-pulse pattern located between the two ON-pulse patterns is madelonger as the length of the record mark to be formed is longer, whereint _(total)*0.45≦t _(top) ≦t _(total)*0.65, where a length of firstapplied one of the two ON-pulse patterns used in forming the record marklonger than the specified length is t_(top), and a sum of the lengths ofthe two ON-pulse patterns used in forming the record mark longer thanthe specified length is t_(total).
 8. The information recordingapparatus of claim 7, arranged so that when the record mark shorter thanthe specified length is formed, the number of the at least one ON-pulsepattern is one.
 9. The information recording apparatus of claim 7,wherein the specified length is 0.82*λ/NA, where a wavelength of thelaser beam is λ, a numerical aperture of an objective for condensing thelaser beam is NA.
 10. The information recording apparatus of claim 7,arranged so thatnT*0.70≦t _(total) ≦nT*0.85, where the length of the record mark to beformed is nT (T is a clock cycle).
 11. The information recordingapparatus of claim 7, arranged so that, of the two ON-pulse patternsused in forming the record mark longer than the specified length, alength of the OFF-pulse pattern following the last ON-pulse pattern isfrom 0.4T to 0.5T inclusive (T is a clock cycle).
 12. The informationrecording apparatus of claim 7, arranged so that the following conditionis satisfied:λ/NA≦640 nm, where a wavelength of the laser beam is λ, and a numericalaperture of an objective for condensing the laser beam is NA.
 13. Aninformation recording method comprising: modulating a laser beamaccording to a plurality of pulse train patterns each including at leasta recording-power level, a base power level, and a medium power levelhaving a power intermediate between the recording-power and base powerlevels; and irradiating a write-once optical recording medium having abase and a record layer on the base with the laser beam modulatedaccording to the plurality of pulse train patterns to form a pluralityof record marks in the record layer, each of the pulse train patternscorresponding to a record mark in the plurality of record marks, andeach of the plurality of record marks in the plurality of record markshaving a different length, wherein the plurality of pulse train patternsincludes a first pulse train pattern and at least two second pulse trainpatterns, the first pulse train pattern corresponds to a record mark inthe plurality of record marks having a length shorter than a specifiedlength, and includes only one ON-pulse pattern and only one OFF-pulsepattern, each of the second pulse train patterns corresponds to a recordmark in the plurality of record marks having a different length longerthan the specified length, includes only two ON-pulse patterns and onlytwo OFF-pulse patterns, and each of the second pulse train patternsdiffers from other second pulse train patterns in a length of only oneof the two OFF-pulse patterns, each ON-pulse pattern transitions fromone of the base power level and the medium power level to therecording-power level, and then transitions from the recording-powerlevel to the base power level, each OFF-pulse pattern includes the basepower level following the ON-pulse pattern, whereint _(total)*0.45≦t _(top) ≦t _(total)*0.65, where a length of firstapplied one of the two ON-pulse patterns used in forming the record marklonger than the specified length is t_(top), and a sum of the lengths ofthe two ON-pulse patterns used in forming the record mark longer thanthe specified length is t_(total).
 14. The method of claim 13, whereineach of the second pulse train patterns differs from the other secondpulse train patterns in a length of the OFF-pulse pattern locatedbetween the two ON-pulse patterns.